Japanese unexamined patent application publication No. 2004-140548 discloses a portable terminal device that is mounted in a cradle provided in a vehicle. If the portable terminal device is removed from the cradle without authorization, the portable terminal device treats input signals from an input operation unit of the portable terminal device as invalid. The portable terminal device is determined to have been removed from the cradle without authorization if a prescribed time elapses after the portable terminal device is removed while the key is not inserted into the ignition of the vehicle. When the portable terminal device has been removed without authorization, all input signals from the input operation unit are invalid, except for a predetermined password. The authorized operator of the vehicle can re-enable input operations by inputting the predetermined password on the input operation unit of the portable terminal device.
However, since the portable terminal device disclosed in Japanese unexamined patent application publication No. 2004-140548 is configured of a cell phone, the authorized operator of the vehicle may store personal information in a memory section of the cell phone. In this case, any non-authorized operator possessing a key to the vehicle can access the personal information stored in the memory section without permission simply by removing the portable terminal device from the cradle while the key is inserted in the ignition. In other words, any person possessing a key to the vehicle can view confidential data, such as the personal information stored in the memory section of the portable terminal device, without authorization. Hence, the portable terminal device disclosed in Japanese unexamined patent application publication No. 2004-140548 does not entirely prevent unauthorized access of the portable terminal device and, thus, the original problem remains unresolved.